Such machines are known for example from U.S. Pat. Nos. 5,062,359 and 5,899,145 and from International Patent Application WO 03/047862. In order to facilitate maintenance operations, some essential parts of the printing machine, needing careful maintenance, are mounted on a mobile carriage, which can be displaced from a working position to a maintenance position upon activation of a control means. In the case of WO 03/047862, two such mobile carriages are provided. Further examples of printing machines with movable carriages are disclosed for instance in European patent applications EP 0 132 858, EP 0 132 859, EP 0 343 104, EP 0 343 105, EP 0 343 106, EP 0 343 107, EP 0 351 366 or EP 0 415 881.
European patent application EP 0 444 227 discloses a safety device for a printing machine wherein first and second pressure-sensitive mat switches are disposed in the path of a mobile inking carriage. The first pressure-sensitive mat switch is disposed below the mobile carriage in the state where this latter carriage is coupled to the printing unit of the printing machine. This first mat switch is adapted to be turned on if an operator stands between the printing unit and the mobile inking carriage when this latter has been retracted. The second pressure-sensitive mat switch is disposed below the mobile inking carriage in the state where this latter carriage is retracted away from the printing unit. This second mat switch is adapted to be turned off if an operator stands behind the mobile inking carriage when this latter is coupled to the printing unit. While the disclosed safety device permits detection of the presence of an operator in the displacement path of the mobile inking carriage, the inherent construction of the safety device implies that the operator must be physically present in the path of the mobile inking carriage in order to trigger the safety device. While, this safety device might be sufficient for low displacement speeds of the mobile carriage (which speeds are usually of the order of 1 meter per minute) as there will be sufficient time for the system to react to the presence of an operator in the path of the mobile carriage, there remains a risk that the system will not react quick enough upon entrance of an operator in the path of the mobile carriage when this latter is displaced at greater speeds. This is especially critical with the safety device of EP 0 444 227 as triggering of the system only occurs upon trespassing of the operator on one of the pressure-sensitive mat switches. A hand or arm of the operator might for instance get caught by the mobile carriage before the operator actually sets a foot on the pressure-sensitive mat switch. An improved solution from the point of view of safety is thus required. In addition, the safety device of EP 0 444 227 actually requires two separate pressure-sensitive mat switches in order to operate adequately, i.e. check for the presence of an operator both in the coupled state and the retracted state of the mobile carriage.
Although human operators are warned of the danger to enter the access zone during displacement of said mobile carriage or carriages, it appeared that security had to be improved.
Further, a need has arisen for a machine with a mobile carriage than can moved more quickly from the working position to the maintenance position without this being detrimental to the security of the human operators. In that respect, in the machines of the prior art, it was a common procedure to limit the displacement speed of the mobile carriage to low levels so as to reduce as much as possible the risk of human operator being caught in the path of the mobile carriage.